The present invention relates to video signal processing and more particularly to apparatus for deriving a burst gate signal from composite video signal.
The burst gate signal is nominally a gating signal which occurs during all or a part of the chrominance reference burst interval on each horizontal line of video signal.
Burst gate signals are employed in e.g. color television receivers, for deriving automatic chrominance control signals, reference frequency signals or clock signals, phase locked to the burst component of the composite video signal, etc.
In the NTSC composite video signal the burst signal consists of 8 to 12 cycles of 3.58 MHz signal located on the "back porch" of the horizontal synchronization pulse. The beginning of the burst interval is located 0.38 microseconds or more from the trailing edge of the horizontal synchronization pulse. Conventional methods for generating a burst gate pulse involve timing an interval from either the leading or trailing edge of the horizontal synchronization pulse and at the end of the interval enabling a pulse generator such as a one-shot multivibrator. Such systems produce burst gate pulses that are always substantially equidistant from the horizontal synchronization pulse. However, burst signals on video signals from different channels or from non-standard sources such as video tape recorders are not necessarily equidistant from the horizontal synchronization pulse. In a particular receiver the burst gate pulse may, therefore, not frame like intervals of the burst signal from different channels which may result in non-uniform system performance from channel-to-channel.
It is an object of the present invention to generate burst gate pulses that will occur during like portions of the burst interval regardless of the source of video signal.